Variable delay lines that have been used in commercial radio communication devices in recent years are a bandpass filter that uses the transmission and receiving frequency band of the radio communication device as a signal passband. The variable delay lines are characterized in that the coupling capacitance of the bandpass filter is changed to change the passband thereof to change the absolute value of the delay time that the variable delay line has (hereinafter referred to as “absolute delay time”).
As shown in FIG. 15, a conventional variable delay line 100 is arranged such that capacitors 106, 108 and a variable-capacitance capacitor 110 are connected in series between an input terminal 102 and an output terminal 104, and first and second resonant circuits 112, 114 are connected between one and other terminals of the variable-capacitance capacitor 110 and ground (see Japanese Laid-Open Patent Publication No. 2001-119206).
When a certain input signal is supplied from the input terminal 102 of the variable delay line 100, the output terminal 104 outputs an output signal having a predetermined absolute delay time shown in FIG. 16. If the coupling capacitance C of the variable-capacitance capacitor 110 shown in FIG. 17 is changed, then the absolute delay time changes as shown in FIG. 18. For example, if the coupling capacitance C is changed from C1 to C2 or C3 (C1>C2>C3), then the absolute delay time increases as shown by the different curves in FIG. 16. If the coupling capacitance C has a wider adjustable range, then the absolute delay time of the variable delay line 100 has a wider variable range (hereinafter referred to as “variable delay time”).
If the coupling capacitance C is reduced, then the bandwidth of the passband of the variable delay line 100 becomes narrower, attenuating the frequency response of the variable delay line 100 shown in FIG. 17 and increasing the return loss shown in FIG. 18.
If the coupling capacitance C is changed, then the capacitor 106 and the first resonant circuit 112 near the input terminal 102 shown in FIG. 15 and the capacitor 108 and the second resonant circuit 114 near the output terminal 104 are brought out of balance thereby varying the values of the input and output impedances of the variable delay line 100. Thus, it is difficult to achieve impedance matching on the variable delay line 100. Therefore, the return loss shown in FIG. 18 increases. Furthermore, the frequency response shown in FIG. 17 are greatly attenuated, resulting in an increased insertion loss over the variable delay line 100 and an increased deviation of the absolute delay time shown in FIG. 16.
Under these circumstances, even when the variable delay line 100 shown in FIG. 15 is connected to other electronic components through the input terminal 102 and the output terminal 104, it is difficult to achieve impedance matching between the variable delay line 100 and the other electronic components. Consequently, the insertion loss over the variable delay line 100 and the radio communication device further increases. Since the above deviation is large, the passband of the variable delay line 100 is reduced, resulting in a significant distortion in the output signal output from the output terminal 104.
For example, if the variable delay line 100 is adapted in an actual radio communication device, then the variable delay line 100 requires a variable delay time of at least about 0.5 ns. However, inasmuch as the frequency response are reduced and the return loss is increased with a variable delay time of about 0.4 ns, as shown in FIGS. 16 through 18, it is difficult for the variable delay line 100 to have desired characteristics.